1. Field of the Invention
The present invention relates generally to synthetic aperture radar (SAR) and, more particularly, to a method and architecture to maximize the measurement potential of a SAR using a partial polarimetric scheme by obtaining the four Stokes parameters of a backscattered field.
2. Description of the Related Art
In many applications using space-based synthetic aperture radar (SAR), the prime objective is to maximize the measurement potential thereof in response to backscatter from a random field whose elements have unknown orientation relative to the polarity of the radar's illumination. Measurement potential is maximized, if and only if, the data products are the four Stokes parameters of the backscattered field (or their logical equivalent) (see R. K. Raney, “Dual-Polarized SAR and Stokes Parameters,” IEEE Geoscience and Remote Sensing Letters, vol. 3, pp. 317-319, 2006, which is incorporated by reference herein in its entirety). Rotational invariance implies that the transmitted polarization must be circular. The choice of polarization basis for the receiver is open to optimization of the radar's design within reliability, mass, and power constraints.
The high-level objective of “partial polarimetry” or “compact polarimetry” (these terms are used interchangeably herein) is to exploit coherent dual-polarized radar data to realize many of the benefits of a quadrature-polarized system without the attendant costs, such as halved swath coverage and doubled average transmitter power. It should be stressed that any partial-polarimetry scheme is not an equivalent substitute for quadrature polarization. Partial polarimetry simply is a major and efficient step up from a single channel system towards full polarimetric measurement capabilities. Partial polarimetry is a reasonable strategy when system resources (power, mass, data volume, or cost) preclude full polarimetry. Partial polarimetry is also compatible as an optional mode for any radar that implements full polarimetry.
One of the first partial polarimetric concepts was the π/4 mode, which posits radiating a linearly-polarized field at 45° (with respect to either horizontal (H) or vertical (V) orientations), then receiving coherently the resulting H and V backscatter components. The concept subsequently was extended to include circularly polarized transmission, with the suggestion that the two types of transmitted polarization should lead to equivalent results. It is now generally accepted that these two transmit polarizations lead to different results (see R. K. Raney, “Decomposition of hybrid-polarity SAR data,” presesnted at POLinSAR, Frascati, Italy, 2007, which is incorporated herein by reference in its entirety). For clarity, as used herein “hybrid-polarity” refers to the “circular transmit, H&V linear receive” case, in contrast to the “π/4 mode”, which refers to the “slant-linear transmit, H&V linear receive” case. Both of these mixed-polarity modes have been used extensively in meteorological radar.
In the π/4 mode, decomposition analyses are most successful for scatterers within the scene whose orientation distributions are predominantly horizontal or vertical. In contrast, however, the objective of many applications is to ascertain the prevailing orientation of backscattering constituents, rather than to presume them at the outset. For such applications, an a priori assumption on backscatterer orientation is not appropriate. Further, there are important applications, such as planetary geology, in which decomposition should be able to classify dihedral-like backscattering features regardless of their orientation. Such applications are best served by rotational invariance, which requires that the illumination be circularly polarized. In short, the preferred form of compact polarimetry depends on the intended application.
An alternative compact polarimetric approach is to receive coherent dual-circular polarizations in response to a circularly-polarized transmitted field. End-to-end circular polarization has an extensive heritage in radar astronomy, from which very good results have been obtained, including backscatter analyses based on Stokes parameters. A spacecraft SAR that is circularly-polarized on transmit and dual-circularly-polarized on receive would be a viable alternative to the hybrid-polarity architecture described herein, but only if the required hardware were more appealing and the resulting measurements were comparably robust. However, such an embodiment implies several disadvantages in hardware and performance that are avoided by the hybrid-polarity method of this invention.
Circular polarization is not entirely foreign to Earth-observing SARS. Noteworthy examples include analysis of quadrature-polarized data from sloping terrain, for which it has been shown that synthesis of circularly-polarized data leads to results that are superior to those from the more conventional linearly-polarized data construct. However, in all such cases, the starting point requires data generated by a quadrature-polarized system, thus invoking all of the attendant disadvantages of that mode.
Recent studies conducted in support of two radars being implemented for deployment at the Moon have looked carefully at alternative architectures, within the constraints of small lunar orbiters. The science requirements include measurement of the circular-polarization ratio, maximized potential to distinguish between backscatter types, and robustness to randomly-oriented dihedral backscatterer distributions. The implementation requirements include minimal mass and power.
Given a SAR constrained to transmit only one polarization, what is needed are a method and architecture that meet two aggressive measurement objectives: (i) full characterization and exploitation of the backscattered field, and (ii) invariance to geometrical orientations of features in the scene.